Friction stir welding is a relatively new technology that has been developed for welding aluminum alloys and other materials. The friction stir welding process generally involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin or spindle. Force is exerted to urge the spindle and the workpieces together, and frictional heating caused by the interaction between the spindle and the workpieces results in plasticization of the material on either side of the joint. The spindle is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing spindle cools and solidifies to form a weld.
It will be appreciated that large forces must be exerted between the spindle and the workpieces in order to apply sufficient pressure to the workpieces to cause plasticization of the material. For instance, for friction stir welding an aluminum alloy plate of 1/4-inch thickness, forces of up to 4000 pounds or more may have to be exerted between the spindle and the plate. In a conventional friction stir welding process, these large forces are absorbed at least partially by a back-up member which engages the workpieces on the "back side" of the weld opposite from the spindle. Where the workpieces have sufficient structural strength and rigidity, part of the welding forces may be absorbed by the workpieces themselves. However, in many cases the workpieces are semi-flexible structures which are incapable of supporting and absorbing the large forces involved in a friction stir welding process. Accordingly, the back-up member is usually supported by a substantial support structure.
A number of challenges are presented in friction stir welding a hollow cylindrical workpiece. Because of limited space inside the workpiece, the rotating friction stir welding tool generally must engage the workpiece from the outside and a suitable back-up tool must support the inner surface of the workpiece along its entire circumference to counteract the large inward forces exerted on the workpiece by the welding tool. A one-piece or fixed geometry back-up tool is impractical, and could not be used in workpieces in which the opening in the workpiece through which the back-up tool must be inserted is smaller in diameter than the portion of the workpiece to be welded. Thus, the back-up tool must be constructed from a plurality of members that can be placed inside the workpiece and then assembled into a full-circumference back-up tool.
The multi-component construction of the back-up tool is not optimum from the standpoint of rigidity of the tool. Tool rigidity is important because, unless the back-up tool has sufficient rigidity, the welding forces can cause deformations of the workpiece, leading to problems such as irregular welds. Accordingly, one challenge in friction stir welding hollow cylindrical workpieces is providing a back-up tool that can fit through an opening in the workpiece and can be assembled into a full-circumference back-up tool having sufficient rigidity to prevent excessive deformation of the workpiece during welding. The back-up tool should also be capable of being assembled and disassembled relatively quickly.
Another problem encountered in friction stir welding a cylindrical structure along a circumferential weld path is that the heat generated during the welding process tends to cause radial growth of the structure through thermal expansion. As a result, the welding tool tends to become buried in the weld metal, causing excessive metal flash and voiding.
Still another problem in friction stir welding a cylindrical structure relates to the rotational driving of the structure. The friction stir welding tool remains in one place and the cylindrical workpiece is rotatably driven about its axis to cause the welding tool to traverse a circumference of the workpiece. Prototype welding equipment developed by the assignee of the present application employed a rotary drive mechanism that drove the workpiece by means of an arm that engaged the workpiece and rotated about an axis coinciding with the axis of the workpiece. Thus, the torque arm of the drive mechanism was essentially equal to the radius of the workpiece. It will be appreciated that for large-diameter workpieces, the resulting torque requirement for the drive mechanism could be quite large. For instance, assuming a horizontal welding load of 2000 pounds that must be overcome by the drive mechanism, a 16-foot diameter workpiece would require a drive torque of 16,000 foot-pounds.
A further drawback of the prototype center-drive mechanism is that the drive arm tended to flex, which caused imprecise control of the rotational motion of the workpiece. For instance, at the end of a weld when the drive mechanism was stopped and the weld tool was withdrawn from the workpiece, the return of the drive arm to a relaxed condition resulted in some further rotational movement of the workpiece, causing an elongation in the exit "keyhole" formed by the withdrawal of the weld tool. Additionally, when starting the drive mechanism to begin a welding operation, the flexing of the drive arm resulted in some backlash such that movement of the workpiece did not begin precisely when commanded and the speed of the workpiece was not as uniform as desired. These problems were noted in welding 3-foot diameter tanks. With larger-diameter structures, the problems caused by drive arm flexure likely would be even greater.